Process for the melting down and remelting of materials for the production of homogeneous metal alloys

ABSTRACT

In the case of a process for the production of homogeneous mixtures of alloys, in particular of intermetallic phases of at least two alloy components, by the melting of raw materials in an inductively heated cold wall furnace the following processing steps are applied:  
     a) in a first processing step, the alloy components are melted into blocks with predetermined alloy composition according to the amount, and  
     b) in a subsequent processing step, at least one of the blocks from the first processing step is melted down in an inductively heated cold wall furnace arrangement ( 60 ) where the melt is stirred by the electromagnetic field energy fed into the melt in such a manner that its alloy components are mixed thoroughly in such a manner that the melt ( 55 ) obtains a homogeneous material composition over its entire volume.  
     Optionally, the first processing step can be carried out in an inductively heated cold wall furnace arrangement which is charged with chargeable raw materials, or the first processing step can be carried out by a vacuum arc remelting process in a cold wall furnace arrangement which is charged with preformed consumable electrodes.

[0001] The invention relates to a process for the production of alloys according to the preamble of Claim 1.

[0002] The invention concerns itself in particular with the melting and remelting of reactive, refractory metals and alloys in a cold-wall furnace oven in a vacuum and/or an atmosphere of inert gas, preferably at vacuum pressures<10⁻¹ mbar. These melting processes serve to produce homogeneous metal blocks from chargeable raw materials.

[0003] For this, a production process is known in which the raw material, which can also be presented in powdered form as well as lumpy, is first of all pressed in a definite mass composition into individual bars. The appropriate amount of the individual fractions of alloys is selected according to the desired mass composition of the individual bars. These pressed and compressed bars are joined to one another to form an electrode which is used as the melting electrode in a vacuum arc remelting process. The consumable electrode is remelted thereby. Thereby, the fractions of alloys are mixed through still further in the liquid melt. The melt is subsequently drawn off as a block for further processing. According to the homogeneity required, it has been shown to be necessary to remelt this block as a consumable electrode in a further process. Since during a single remelting process no complete alloy homogeneity can be achieved over the length of the block, the remelting process must be repeated multiple times according to the required homogeneity of the desired alloy. The entire processing time for the single vacuum arc remelting process consists of the charging and melting times and is ca. 12-18 h.

[0004] A disadvantage of this process is that the material preparation, in particular processing of the consumable electrode, sometimes requires a time-intensive and cost-intensive expenditure of effort. In particular, under the requirement of a predetermined homogeneity of the melted alloy, the block to be produced must be remelted repeatedly, which, taking into account the aforementioned required processing times, means a clear loss of productivity, because each electrode must unavoidably be remelted into a block of greater diameter.

[0005] The objective of the invention is thus to specify a process of the class described initially by which alloys can be produced with extraordinarily homogeneous distribution of the alloy components over the entire volume.

[0006] The realization of the objective set is accomplished with the process according to the invention described initially by the features in the characterization of Claim 1.

[0007] In the process according to the invention, the subject is a melting technology by which it is insured that, starting from the individual alloy components with different densities, conditions (history of its origin, lumpiness), and melting points, a desired alloy is produced with exact chemical composition. Contrary to the previous experience with pure vacuum arc remelting processes, it has been shown that, by adhering to the melting sequence according to the invention, an exact chemical composition of an alloy reproducible in high quality, that is, with a homogeneity prevailing over the entire volume of the final melt, can be produced. The problem of the chemical inhomogeneity in the case of remelting in a pure vacuum arc remelting process of the type described above is solved thereby in a simple manner. The kernel of the invention consists of the fact that, in contrast to the prior art in remelting, the stirring motion, and thus the mixing process in the melting pool of the cold wall induction furnace, is used advantageously for through mixing of the melts and uniform distribution of the alloy elements in the melt.

[0008] In practice it has been proven that the mixing through of the melts within the melt pool of the cold wall induction furnace is sufficiently effective.

[0009] In the case of an advantageous process management, the alloy components are introduced in a first processing step as chargeable material which leads to a predetermined alloy composition via a lock chamber directly into a charging area of a cold wall induction furnace. After melting down the material, it is mixed thoroughly in the melt pool by the agitating field induced by the induction field. Thereby a homogenized melt arises which can be drawn off continuously as rigid blocks from the cold wall induction furnace via an apparatus for drawing off blocks.

[0010] The process according to the invention is suited in particular for the production of alloys which consist of refractory and/or reactive metals such as, in particular, alloys containing titanium or titanium compounds. For the charging of the cold wall furnace, the raw material is presented either as lumpy material and/or as powder and/or as a granulate. This raw material is pressed for the first remelting either into solid blocks which can be used as material for a vacuum arc remelting process used optionally for block production, or it is introduced via a material lock directly into a cold wall induction furnace as described above.

[0011] In all, a clear reduction with regard to the expenditure for the preliminary treatment and subsequent treatment of the melt material results from process management according to the invention as well as from the use of the cold wall induction furnace for the production of homogeneous alloys.

[0012] Additional advantageous developments of the process according to the invention follow from the subordinate claims.

[0013] The object of the invention will be explained in more detail in the following with the aid of a particularly preferred embodiment example represented in the figures.

[0014]FIG. 1 is the axial section through a cold wall furnace arrangement with a layered charge in the operational state for the first melt for the production of the material for the second melt;

[0015]FIG. 2 is a cold wall furnace arrangement for the generation of the second melt;

[0016]FIG. 3 is an assembled melt electrode; and,

[0017]FIG. 3b is a remelted, partially homogenized block of material.

[0018] In FIG. 1, a cold wall furnace arrangement 2 is represented which consists of a slotted furnace wall 3 in the form of a water-cooled hollow body. The management of the cool water is not represented for simplicity's sake. It is however also possible to replace the coolant water by another cooling medium. The furnace wall 3 is encircled by an induction coil arrangement 7 which supplies the necessary heating and meltings as well as stirring energy. The power supply unit for the induction coil arrangement 7 is likewise not represented. Since the construction principle of a cold wall furnace with induction coil, taken in itself, is the state of the art, entering into it any further would be superfluous.

[0019] It is merely maintained that the induction coil arrangement 7 is equipped with a greater winding number and can be subdivided into individual partial coils 20 a, 20 b, 20 c, 20 d, 20 e which can be attached to power supply units independently of one another. These can then be regulated or controlled separately of one another in order to be able to set the heating power and the stirring power via the height of the furnace wall 3.

[0020] The entire cold wall furnace arrangement 2 is situated with its lower furnace flange 16 on positionally fixed lower supports 24 a, 24 b. On the lower furnace flange 16, the furnace wall 3 encircled by the induction coil arrangement 7 is situated with surrounding lower sealing elements 23 sealing vacuum-tight. The upper furnace flange 14 is situated above on the furnace wall 3. Between the upper furnace flange 14 and the furnace wall 3, an upper sealing element 15 situated in a encircling slot is provided which forms a vacuum-tight connection between the furnace wall 3 and the upper furnace flange 14. The furnace wall 3 and the upper and lower furnace flange 14 and 16 are disposed coaxially to one another and surround a vertically aligned passageway zone for the material to be melted. For charging, the cold wall furnace arrangement 2 has a material lock 4 above the upper furnace flange 14 which can be sealed vacuum-tight with a lock opening 10 with respect to the outer space. The material 9 to be alloyed is introduced via the lock opening 10 into the lock chamber 11 where, according to the alloy desired, the alloy fractions are fed together according to the amount in the appropriate ratio in the lock chamber 11. The alloy material 9 to be melted is gathered together in the charge material space 34 of the passageway zone of the furnace wall 3 and migrates according to the degree of liquefaction of the entire alloy material 9 into the actual melt zone which forms the melt pool 32. The axial position of the melt pool 32 is fixed by the arrangement of the induction coils 20 a-20 e via which the necessary melting and stirring energy are fed into the melt inductively. The stirring motion of the melt being formed within the melting zone 32 is represented by the arrow U pointing toward its starting point and indicating the direction of the melt eddy. In principle, the invention is not restricted to the eddy arrangement U represented in FIG. 1, but rather, it can be expressed differently in size and direction within the melt zone 32 by suitable selection of the individual coil windings 20 a-20 e.

[0021] The melt is continuously stirred within the melt pool 32 by the stirring motion, whereby the individual alloy components are homogenized in the entire melt collected in the melt pool 32. Adjacent to the melt pool 32 at its lower area there is the hardening zone in which the hardened material block 30 is situated on a supporting foundation 25 which is lowered continuously via a block withdrawal device 6. The directions of motion are indicated by the double arrow Z.

[0022] The previously described remelting process takes place at low pressure of <10⁻¹ mbar. For this, the residual atmosphere located in the cold wall furnace arrangement 2 is evacuated via connecting suction pipes 12 in a Known manner with vacuum pumps not represented in the drawings.

[0023] In order to receive the axially directed forces exercised on the upper furnace flange 14 and the lower furnace flange, the upper furnace flange 14 and the lower furnace flange 16 are fixedly connected to one another with connecting struts 22.

[0024] Subsequently to the process of drawing off of the block represented in FIG. 1, a homogeneous melt is produced by means of a cold wall furnace 60 known in itself. The cold wall furnace 60 represented in FIG. 2 consists essentially of the furnace floor 17 on which the furnace wall 21 is set. The furnace wall consists in a known manner of a palisade arrangement 21, 21′, . . . where, between the individual palisades 21, 21′, . . . , spacings for the engagement of the melt and agitating magnetic field are provided. Sealing elements of an insulating material are customarily located in these spacings. The stirring or melting magnetic field is generated via an induction coil 19 which has individual coil windings 20 a-20 d according to the prior art with power supply devices not represented in FIG. 2. For the generation of a first melt by a vacuum arc remelting process, the alloy components are presented, for example, as powder, as granulated metals, or as lumpy material which can be pressed into a solid pressed block with definite mass composition. These individual blocks 40, 41 (see FIG. 3a) are put together for the formation of a consumable electrode 42 and welded to one another at the connecting seams 50, 52. For the welding of the blocks 40, 41, in particular, an electron beam welding process is provided. The blocks 40, 41, joined together to form a consumable electrode 42, are subsequently first of all melted down in a first vacuum arc remelting process List of reference numbers  2 Cold wall furnace arrangement, cold wall furnace  3 Furnace wall  4 Material lock, material feed  6 Block exit  7 Induction coil arrangement  8 Lower part  9 Alloy material 10 Lock opening 11 Lock chamber 12 Connecting suction pipe 13 Induction furnace 14 Upper furnace flange 15 Upper sealing element 16 Lower furnace flange 17 Furnace floor 18 Insulating sheath 19 Induction coil 20a-e Coil winding 21, 21′, 21″, 21′″, 21″″ Palisades 22 Connecting struts 23 Lower sealing element 24, 24a, 24b Lower supports 25 Support base 26 Apparatus for drawing off blocks 30 Hardened block/block 32 Melt pool, block melt 34 Charge material space 40 Formed material piece 41 Formed material piece 42 Melting electrode 44 Block 50 Joint-seam 52 Joint seam 55 Melt A Input block, premelted F Filling path U Turbulent flow Z Thrust direction

[0025] not represented in the figures, whereby the raw material is distributed homogeneously in the melt up to a certain degree. The melt generated in this manner is subsequently transferred into suitable casting molds in which the melt material hardens to form a block 44 (see FIG. 3b). The volume of the block is chosen so that it fills up the furnace volume of the cold wall furnace 60 represented in FIG. 2.

[0026] For further homogenization of the block 30 from FIG. 1 or the block 44 according to FIG. 3b, said block is transferred into the cold wall furnace 60 according to FIG. 2 and subsequently, the oven chamber encircling the cold wall furnace 60 and not represented is closed and evacuated to a typical operating pressure of 10⁻¹ mbar, and the electrical power of the induction coil arrangement 19 is switched on. After liquefaction of the block 30, the melt 55 is thoroughly homogenized by the inductive agitating field. It can be molded into a desired semifinished product for cooling. 

1. Process for the production of homogeneous mixtures of alloys, in particular of intermetallic phases of at least two alloy components, by the melting of raw materials in an inductively heated cold wall furnace, characterized by the following processing steps: a) in a first processing step the alloy components are melted into blocks (30, 44) with predetermined alloy composition according to the amount, and b) in a subsequent processing step at least one of the blocks (30, 44) from the first processing step is melted down in an inductively heated cold wall furnace arrangement (60) where the melt is stirred by the electromagnetic field energy fed into the melt in such a manner that its alloy components are so thoroughly mixed that the melt (55) contains a homogeneous material composition over its entire volume.
 2. Process according to claim 1, characterized by the fact that the first processing step is carried out in an inductively heated cold wall furnace arrangement (2) which is charged with chargeable raw materials.
 3. Process according to claim 1, characterized by the fact that the first processing step is carried out by a vacuum arc remelting process in a cold wall furnace arrangement which is charged with preformed consumable electrodes (42, 44).
 4. Process according to claim 1, characterized by the fact that the entire volume of the blocks (30, 44) used for the remelting process provided in the inductively heated cold wall furnace (60) is chosen in such manner that its entire volume corresponds to the filling volume of the inductively heated cold wall furnace (60).
 5. Process according to claim 1, characterized by the following processing steps: a) at least one part of the alloy components is pressed into chargeable material (9) with predetermined alloy composition, b) the material (9) is introduced via a lock chamber (11) into a melting pool (32) which is encircled by coil windings (20 a-20 e) of an induction coil arrangement (7), c) the material (9) is heated by the supplying of electromagnetic field energy via an alternating field applied to the coil windings (20 a-20 e) in such a manner that the material (9) is melted down via the magnetic alternating field running in the melting pool (32) where the melt is furthermore mixed through by the induced magnetic alternating field in the melt pool (32), and d) the melt material hardened below the melt pool (32) is drawn off as a block (30) from the cold wall furnace arrangement (2) via a device (6) for drawing off blocks located at the lower end of the induction coil arrangement (7).
 6. Process according to one of the claims 1-5 characterized by the fact that the alloy components are chosen from highly reactive materials, in particular from titanium or titanium compounds.
 7. Process according to one of claims 1-6 characterized by the fact that the raw material is chosen as lumpy material and/or as powder and/or as granulate. 